Image processing apparatuses such as an electronic camera which uses a memory card having a solid-state memory element as a recording medium, and records and plays back still and moving images sensed by a solid-state image sensing element such as a CCD or CMOS have commercially been available.
To sense an image by using a solid-state image sensing element (to be referred to as an image sensing element hereinafter) such as a CCD or CMOS, the image processing apparatus can execute dark noise correction processing by calculation processing using dark image data read out after charge accumulation similar to actual photographing while the image sensing element is not exposed, and actual photographing image data read out after charge accumulation while the image sensing element is exposed. A high-quality image can be attained by correcting the sensed image data for image quality degradation caused by dark current noise generated by the image sensing element, a defective pixel due to a slight scratch unique to the image sensing element, or the like. Image quality degradation can be further reduced by correcting a point scratch by interpolation processing using image data of pixels adjacent to a scratched pixel (defective pixel).
In the following explanation, the phrase “scratched pixel (defective pixel)” means a pixel whose output signal is not normal. In other words, “scratched pixel (defective pixel)” includes a pixel whose output signal is too large because of an abnormal dark current noise signal (white scratched pixel) and a pixel which cannot output a signal (black scratched pixel).
As a conventional method of correcting such scratched pixels, the sensor output is evaluated for a standard charge accumulation time under predetermined conditions in shipping a sensor (image sensing element) from the factory. A scratched pixel is determined based on the evaluation result. The type of scratched pixel (black or white scratched pixel, or the like), the address of the scratched pixel, and the scratch level data are acquired. The scratched pixel is corrected using these data. These data contain the type of scratch, position data (x,y) of the scratched pixel, and the level.
The output level of a scratched pixel, particularly a white scratched pixel (white point pixel) is known to greatly change depending on the charge accumulation time of the image sensing element in photographing. Even a pixel which is not defective in general photographing for a short time (short seconds) abruptly increases its scratched pixel level in photographing for a longer time (long seconds), adversely affecting the image quality. Especially when the above-described dark noise correction processing is not executed, no small scratch is corrected, remarkably degrading the image quality by the defective pixel in long-sec photographing.
The prior art suffers the following problem. A scratched pixel is determined in accordance with the scratched pixel level for long seconds, and correction processing is performed for all pixels. Correction processing is also performed for a pixel which does not require correction processing in short-sec photographing, and excessive correction may degrade the image quality.
To solve this problem, for example, Japanese Patent Laid-Open No. 2001-28713 proposes a technique of sensing a dark image prior to photographing operation, extracting a white scratched pixel from the image, and performing correction processing for only the extracted white scratched pixel. However, the correction processing time cannot be ignored particularly upon an abrupt increase in the number of scratched pixels in long-sec photographing, and greatly influences the release time lag and photographing frame interval. This technique is not a practical solution.
A larger number of scratched pixels to be corrected occupy the memory of the image processing apparatus because pieces of address information of scratched pixels to be corrected are stored. For example, in Japanese Patent Laid-Open No. 11-112879, a dark image is photographed prior to photographing operation, and when defective pixels exceeding the memory capacity are generated in extracting white scratched pixels from the image, the image sensing element is powered off to prevent any abnormal image. At this time, even if the user wants to take a picture, he/she cannot take the picture against his/her will.